Rafael Garcia

Optical properties of highly luminescent zinc oxide tetrapod powders

Zinc oxide tetrapod powders with high luminescent characteristics have been grown by simple evaporation and direct oxidation of zinc. Their structural and optical properties have been studied by transmission electron microscopy and by cathodoluminescence. The legs of the tetrapods have a pure hexagonal structure and form an arrowhead joint at the center. The tetrapod legs exhibit strong band edge and weak green luminescence. Monochromatic cathodoluminescence images show that the band-edge emission originates from the bulk of the legs, while the green emission emanates from the surface, indicative of it being related to oxygen vacancies. Low temperature cathodoluminescence shows a direct correlation of near-band-edge emission with specific sites within the structure.

A novel method to synthesize blue-luminescent doped GaN powders

A new method to synthesize highly luminescent GaN:Mg powders has been developed. This method has a high control over the concentration of magnesium in the final product. The method consists of reacting a high purity (99.999 %) galliummagnesium alloy with ultra-high purity ammonia in a horizontal quartz tube reactor at high temperatures for several hours. Electron microscopy showed that the light-gray powders produced by this method consist of at least two different shaped crystallites; large columnar crystals sized around 10 μm and small platelets crystals between 1 and 2 μm. X-ray diffraction showed that those crystallites have a well defined wurtzite structure. Room temperature photoluminescence (PL) and cathodoluminescence (CL) showed a high intensity blue emission around 2.94 e V (422 nm). At helium temperature the well known Mg-related donor-acceptor pair band was observed at 3.25 e V (380 nm) by PL and CL.

Light Emission from GaN Microcrystals

We have grown high quality undoped and n‐type GaN crystallites by a novel technique based on direct reaction of gallium metal with ammonia using a two step method. Powders produced by this method consist of at least two differently shaped crystallites; large columnar crystals sized around 10μm and small platelets crystals between 1 and 3μm. The crystallites have a well defined wurtzite structure, with an exceptionally strong near band‐edge emission at around 3.342 eV. Yellow luminescence (YL) has been observed in Si‐doped and O‐doped powders but not in undoped powders grown by this method.